Dispersion slope compensating optical waveguide fiber

ABSTRACT

Disclosed is a dispersion compensating and dispersion slope compensating single mode optical waveguide fiber. The refractive index profiles of waveguide fibers in accord with the invention are disclosed and described. These index profiles provide a waveguide fiber having negative total dispersion and negative total dispersion slope so that a standard waveguide fiber is compensated over an extended wavelength range. A telecommunications link using the fiber in accord with the invention is also disclosed and described. A standard fiber to compensating fiber length ratio in the range of 1:1 to 3:1 is shown to give optimum link performance with respect to limiting non-linear dispersion effects.

This is a divisional of U.S. patent application Ser. No. 09/822,168filed on Mar. 30, 2001, the content of which is relied upon andincorporated herein by reference in its entirety, and the benefit ofpriority under 35 U.S.C. §120 is hereby claimed. This applicationfurther claims priority to and the benefit of U.S. Provisional PatentApplication No. 60/193,080 filed Mar. 30, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an optical waveguide fiber,and particularly to an optical waveguide fiber that compensatesdispersion slope in a telecommunications link.

2. Technical Background

Dispersion compensation techniques in telecommunications systems orlinks have been used successfully. A technique useful in links alreadyinstalled is one in which total dispersion (also called chromaticdispersion) is compensated by an appropriately designed waveguide fiberformed into a module that can be inserted into the link at an accesspoint such as an end of the link. The compensating waveguide fiber canbe designed to allow operation in, for example, the 1550 nm operatingwavelength window of a link originally designed for the 1310 nmoperating window.

A disadvantage of compensating with a module is that attenuation andnonlinear penalties are added to the link without increasing the usefullink length. Also some of the refractive index profile designs fordispersion compensation are more difficult to manufacture and havehigher attenuation than the fibers making up the link.

Another dispersion compensation scheme is to include both positive andnegative dispersion fibers in the cables of the link. Each cable cancontain both positive and negative total dispersion waveguide fibers, orthe link can be formed using cables having only positive dispersiontogether with cables having only negative dispersion. The relativelyhigh attenuation and low effective area of the negative dispersion fibercan be a problem in this scheme as it is in the dispersion compensatingmodule solution. Also the cable inventory must be managed carefully,because replacing or repairing a cable involves tracking of anothervariable (the sign of the dispersion of fibers in the cable). In certainprofile designs a mismatch of mode fields between the positive andnegative total dispersion fibers exists and results in excessive spliceor connecting losses.

There is therefore a need for a total dispersion compensating strategyin which the compensating fiber is a part of the link length and theproblem of the compensating fiber producing excess link attenuation isaddressed. Furthermore, a solution that includes introducing negativedispersion cabled fiber into the link must offer a benefit that offsetsthe cost of cable inventory management and that does not introduceexcess splice loss into the link.

A further desired characteristic of a total dispersion compensationsolution is that the compensation be effective over an extendedbandwidth to facilitate use of wavelength division multiplexed linkarchitectures.

DEFINITIONS

The following definitions are in accord with common usage in the art.

The refractive index profile is the relationship between refractiveindex or relative refractive index and waveguide fiber radius.

A segmented core is one that is divided into at least a first and asecond waveguide fiber core portion or segment. Each portion or segmentis located along a particular radial length, is substantially symmetricabout the waveguide fiber centerline, and has an associated refractiveindex profile.

The radii of the segments of the core are defined in terms of therespective refractive indexes at respective beginning and end points ofthe segments. The definitions of the radii used herein are set forth inthe figures and the discussion thereof.

Total dispersion, sometimes called chromatic dispersion, of a waveguidefiber is the sum of the material dispersion, the waveguide dispersion,and the inter-modal dispersion. In the case of single mode waveguidefibers the inter-modal dispersion is zero.

The sign convention generally applied to the total dispersion is asfollows. Total dispersion is said to be positive if shorter wavelengthsignals travel faster than longer wavelength signals in the waveguide.Conversely, in a negative total dispersion waveguide, signals of longerwavelength travel faster.

The effective area is A_(eff)=2Π(∫E² r dr)²/(∫E⁴ r dr), where theintegration limits are 0 to ∞, and E is the electric field associatedwith light propagated in the waveguide.

The relative refractive index percent, Δ%=100×(n_(i) ²−n_(c) ²)/2n_(i)², where n_(i) is the maximum refractive index in region i, unlessotherwise specified, and n_(c) is the average refractive index of thecladding region. In those cases in which the refractive index of asegment is less than the average refractive index of the claddingregion, the relative index percent is negative and is calculated at thepoint at which the relative index in most negative unless otherwisespecified.

The term α-profile refers to a refractive index profile, expressed interms of Δ(b) %, where b is radius, which follows the equation,Δ(b)%=Δ(b_(o))(1−[|b−b_(o)|/(b_(i)−b_(o))]^(α)), where b_(o) is thepoint at which Δ(b)% is maximum, b₁ is the point at which Δ(b)% is zero,and b is in the range b_(i)≦b≦b_(f), where delta is defined above, b_(i)is the initial point of the α-profile, b_(f) is the final point of theα-profile, and α is an exponent which is a real number.

The pin array bend test is used to compare relative resistance ofwaveguide fibers to bending. To perform this test, attenuation loss ismeasured for a waveguide fiber with essentially no induced bending loss.The waveguide fiber is then woven about the pin array and attenuationagain measured. The loss induced by bending is the difference betweenthe two attenuation measurements. The pin array is a set of tencylindrical pins arranged in a single row and held in a fixed verticalposition on a flat surface. The pin spacing is 5 mm, center to center.The pin diameter is 0.67 mm. The waveguide fiber is caused to pass onopposite sides of adjacent pins. During testing, the waveguide fiber isplaced under a tension just sufficient to make the waveguide conform toa portion of the periphery of the pins. The test pertains to macro-bendresistance of the waveguide fiber.

A waveguide fiber telecommunications link, or simply a link, is made upof a transmitter of light signals, a receiver of light signals, and alength of waveguide fiber having respective ends optically coupled tothe transmitter and receiver to propagate light signals therebetween. Alink can include additional optical components such as opticalamplifiers, optical attenuators, optical switches, optical filters, ormultiplexing or demultiplexing devices. One may denote a group ofinter-connected links as a, telecommunications system.

SUMMARY OF THE INVENTION

One aspect of the present invention is a single mode optical waveguidefiber, having a core region and a surrounding clad layer. The referenceto single mode waveguide fiber means that the fiber in cable formusually will carry only a single mode over the range of operatingwavelengths. Persons skilled in the art understand that single modeoperation also includes cases in which more than one mode is propagatedbut that the higher order modes may are strongly attenuated and so donot travel in the waveguide more than a few kilometers. The waveguidefiber in accord with the invention may also be used in a wavelengthrange where a few modes are propagated the full link length and the fewmodes are dispersion compensated. The core region includes at leastthree segments, a central segment beginning at the centerline of thewaveguide fiber, and two annular segments surrounding the centralsegment. In one embodiment, the profile has four segments, a centralsegment, surrounded by a first, second and third annular segment. Eachof the segments is characterized by a refractive index profile, arelative refractive index, and an inner and an outer radius. Therespective segment characteristics are selected to provide a waveguidefiber having a total dispersion at 1550 nm in the range of −30 ps/nm-kmto −60 ps/nm-km and preferably in the range of −30 ps/nm-km to −48ps/nm-km, total dispersion slope at 1550 nm in the range of −0.09ps/nm²-km to −0.18 ps/nm²-km and preferably in the range of −0.09ps/nm²-km to −0.15 ps/nm²-km, an effective area at 1550 nm greater than25 μm², and attenuation at 1550 nm less than or equal to 0.30 dB/km. Ina preferred embodiment, the attenuation at 1550 is less than or equal to0.26 dB/km.

The respective relative indexes, symbolized beginning at the centralsegment as Δ₀, the first annular segment (4 in FIG. 1) Δ₁, and secondannular segment (6 in FIG. 1) Δ₂, are related by the inequalities,Δ_(o)>Δ₂>Δ₁, and Δ₁<0.

In an embodiment of the single mode optical waveguide fiber in accordwith the invention, the central segment has relative index percent inthe range of 0.8% to 1.4% and preferably in the range 0.9% to 1.2%, thefirst annular segment has relative index percent in the range of −0.3%to −0.5% and preferably −0.35% to −0.45%, and the second annular segmenthas relative index percent in the range of 0.20% to 0.45%. Therespective radii associated with this embodiment are for the centralsegment an inner radius zero and outer radius, r_(o), in the range 1.8μm to 3.0 μm, for the first annular segment an inner radius r_(o) andouter radius in the range r_(o)+1.5 μm to r_(o)+3.0 μm, and for thesecond annular segment a center radius in the range 4.5 μm to 10 μm anda width, measured between two points defined by the intersection of thesecond annular segment refractive index profile with a horizontal linedrawn at the half relative index percent value of the second annularsegment refractive index profile, in the range of 0.3 μm to 4.0 μm.

In another embodiment in accord with the invention, the central segmentof the single mode optical waveguide fiber includes a SiO₂ layer at theinterface of the central segment and the first annular segment. ThisSiO₂ layer is no thicker than 1.5 μm. The composition of the layerranges from pure SiO₂ to 90% SiO₂.

In a further embodiment of the waveguide fiber profile, there is aflattened region of refractive index beginning at the outer radius ofthe first annular segment. The width of this region is no greater than5.0 μm.

In yet another embodiment in accord with the invention, the clad layeradjacent the core region has a refractive index less than that of SiO₂.This portion of the clad layer has a thickness no greater than 20 μm.For most refractive index profile designs of optical waveguide fibers,no light is present at a radius about 20 μm greater than the coreradius.

A second aspect of the invention is a telecommunications link whichmakes use of two types of waveguide fibers. A first waveguide type haspositive total dispersion and positive total dispersion slope. A secondtype, made in accord with the invention, has negative total dispersionand negative total dispersion slope. Combining the two fiber types in alink allows one to compensate for accumulated positive dispersion in thefirst waveguide type by using, in the link, an appropriate length ofnegative total dispersion waveguide fiber. The difference in sign of therespective slopes of the first and second waveguide types provides fortotal dispersion compensation over an extended range of operatingwavelengths. In addition, the negative dispersion waveguide fiber canprovide a net negative dispersion in each span which mitigates nonlinearpenalties due to modulational instability and four wave mixing. Thisaccumulated negative dispersion is then compensated periodically bysingle spans of the positive dispersion waveguide fiber.

The link includes a transmitter that provides light signals, a receiverthat receives the light signals, and at least two lengths of opticalwaveguide fiber optically coupled between the transmitter and receiverto transport the light signals. At least one of the waveguide fiberlengths has positive total dispersion and total dispersion slope. Atleast one of the waveguide fiber lengths has negative total dispersionand negative total dispersion slope. The length, total dispersion, andtotal dispersion slope of the fibers are chosen to provide a link lengthhaving a magnitude (as used herein, magnitude refers to absolute valueof either a positive or negative total dispersion or total dispersionslope) of total dispersion and total dispersion slope less than 10ps/nm-km and 0.01 ps/nm²-km, respectively. The combination of fibershaving total dispersion of different sign serve to reduce or eliminatethe signal dispersion. Because the fibers also have total dispersionslope of different sign, the canceling of signal dispersion takes placeover an extended wavelength range.

In an embodiment of the link, the signal dispersion cancellation iseffective over a wavelength range 1280 nm to 1650 nm so that theoperating window includes wavelengths near 1310 nm as well as the C-band(1530 nm to 1565 nm) and L-band (1565 nm to 1650 nm). The dispersiondata show that operation over this very wide wavelength band ispossible.

In another embodiment of the link, the optical waveguide fiber havingpositive total dispersion and total dispersion slope is longer than theoptical waveguide fiber having negative total dispersion and totaldispersion slope. A preferred embodiment is one in which the positivetotal dispersion fiber is at least twice as long as the negative totaldispersion fiber. Because of the characteristics of the refractive indexprofile of the negative total dispersion fiber, this fiber generallyexhibits a higher attenuation relative to that of the positive totaldispersion fiber. Therefore, the link attenuation is reduced whendispersion compensation can be achieved using a shorter length ofnegative total dispersion fiber.

In a further embodiment of the invention, the link is constructed sothat the negative total dispersion fiber is farthest from thetransmitter. The advantage of this construction is due to the highereffective area of the positive total dispersion waveguide compared tothe effective area of the negative total dispersion waveguide.Non-linear dispersion effects, such as cross phase modulation and fourwave mixing, are known to depend upon the ratio of power density in thewaveguide fiber to effective to effective area of the fiber. By placingthe higher effective area waveguide fiber nearest the transmitter, thehigher power signal propagates in the larger effective area fiber. Thesignal is attenuated before reaching the lower effective area, negativetotal dispersion fiber so that the non-linear dispersion effects arekept to a minimum.

In telecommunications links designed for two way signal propagation inthe waveguide fiber, the non-linear effects are minimized by placing thelower effective area waveguide fiber in the center of the link betweentwo segments of the link constructed of the higher effective areawaveguide fiber.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate various embodimentsof the invention, and together with the description serve to explain theprinciples and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a refractive index profile that exhibitsthe main features of the invention.

FIG. 2 is an illustration of a refractive index profile of an embodimentof the invention.

FIG. 3 is an illustration of a refractive index profile of an embodimentof the invention.

FIG. 4 is a chart showing the comparison between a target refractiveindex profile and a measured refractive index profile of a fibermanufactured in accord with the invention.

FIG. 5 is a chart of total dispersion at 1550 nm versus total dispersionslope at 1550 nm and total dispersion at 1550 nm versus attenuation at1550 nm for waveguide fibers made in accord with the invention.

FIG. 6 is a chart showing total dispersion versus operating wavelengthfor a waveguide fiber manufactured in accord with the invention.

FIG. 7 is a chart showing total dispersion slope versus operatingwavelength for a waveguide fiber manufactured in accord with theinvention.

FIG. 8 is an illustration of a compensated link total dispersion versuswavelength in which the link includes waveguide fibers havingcharacteristics in accord with the invention incorporated at a 2:1 ratioof positive total dispersion fiber to negative total dispersion fiber.

FIG. 9 is a chart showing total dispersion versus operating wavelengthfor a waveguide fiber to be compensated by a waveguide fibermanufactured in accord with the invention.

FIG. 10 is a chart showing total dispersion slope versus operatingwavelength for a waveguide fiber to be compensated by a waveguidemanufactured in accord with the invention.

FIG. 11 is an illustration of the relative performance of waveguidefiber designed to compensate link total dispersion.

FIG. 12 is an illustration of a refractive index profile of anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. An exemplary embodiment of the waveguide fiberrefractive index profile of the present invention is shown in FIG. 1.The refractive index profile includes a core region, which begins at thewaveguide fiber centerline and ends at point 18 where the final segmentof the core abuts the surrounding clad layer 20. Central segment 2 has apositive relative index percent, Δ₀%, an inner radius of 0 and an outerradius 12 measured from the centerline to the point at which thedescending portion of segment 2 crosses the horizontal axis, i.e., Δ%equal to zero. First annular segment 4 has a negative relative indexΔ₁%, inner radius 12 and outer radius 14 measured from the centerline tothe point at which the ascending portion of segment 4 crosses thehorizontal axis. Second annular segment 6 includes a flattened regionhaving inner radius 14 and width 10, measure from inner radius 14 to thepoint at which the ascending portion of segment 6 begins, and a regionof raised index having center radius 16 and width 8. The center radius16 is measured from the centerline to the point at which the raisedindex is a maximum, that is, at the relative index point Δ₂%. For thoseprofiles having a flattened segment 6, the center radius 16, such asthat shown in FIG. 12, the radius 16 is measured from the centerline tothe center of width 8. The width 8 is measured between the points ofintersection of the half maximum relative index line and the ascendingand descending portions of the raised index region.

These definitions of radius described in terms of FIG. 1 are appliedconsistently to FIGS. 1-4 and 12 which are all illustrations ofrefractive index profiles in accord with the invention. In the interestof brevity and clarity the corresponding parts of the profiles in eachof FIGS. 1-4 and 12 will have the same number and the definitions of theradii and widths will not be repeated.

The shape of the central segment in FIGS. 1-4 is described as being arounded trapezoid. More particularly, the term rounded trapezoid refersto a central segment having an inner portion beginning at or near thecenterline and having a first slope and an adjacent second portionhaving a second slope. The slope of the second portion is greater thanthat of the first portion. The slope of the first portion is in therange 0 to −0.2/μm. In FIG. 12, the shape of the central segment isrounded and can be described as an α-profile having α in the range of 1to 3.

EXAMPLE 1

A refractive index profile was modeled using the Δ% values and radiusvalues shown in FIG. 1. Central segment 2 has Δ₀% of 1.05%, outer radius1.8 μm, and a rounded trapezoidal shape; first annular segment 4 has Δ₁%of −0.4%, outer radius 4.0 μm, and a rounded step shape; second annularsegment 6 has a flattened region of width 0.9 μm and relative index nearzero (the slight rise in the index of the flattened region is due todopant diffusion and is accounted for in the model calculations), and araised index region Δ₂% of 0.3%, center radius, 5.6 μm, width 0.85 μm,and a symmetrical rounded shape This shape can be generated using theproper α in the α-profile equation set forth above.

The modeled properties of this profile are effective area 26 μm², totaldispersion slope −0.11 ps/nm²-km at 1550 nm, total dispersion −39ps/nm-km at 1550 nm, attenuation 0.233 dB/km at 1550 nm, fiber cut offwavelength 1426 nm, and pin array bend loss 4.1 dB.

The modeled waveguide fiber has respective total dispersion slope andtotal dispersion of about 2× that of standard step index single modeoptical waveguide fiber, having a dispersion zero in the 1310 nmoperating window, but of opposite sign. Using a length ratio ofapproximately 2:1, the waveguide fiber of this example can be used tocompensate the total dispersion slope of standard single mode opticalwaveguide fiber, while yielding a residual negative total dispersion.The effective area of the compensating fiber is reasonable and theattenuation and pin array loss is excellent.

COMPARATIVE EXAMPLE 2

A second refractive index profile was modeled using the Δ% values andradius values shown in FIG. 2. Central segment 2 is identical to that ofFIG. 1, annular segment 4 has Δ₁% of −0.4%, outer radius 4.6 μm, and atrapezoidal shape having a linear portion of positive slope that beginsat Δ₁% and ends at a relative index of −0.3%; second annular segment 6has a flattened region, 10, of width 0.33 μm and relative index nearzero, and a raised index region Δ₂% of 0.4%, center radius, 5.8 μm,width 1.0 μm, and a symmetrical rounded shape. This shape can begenerated using the proper α in the α-profile equation set forth above.

The modeled properties of this profile are effective area 25 μm², totaldispersion slope −0.16 ps/nm²-km at 1550 nm, total dispersion −36ps/nm-km at 1550 nm, attenuation 0.234 dB/km at 1550 nm, fiber cut offwavelength 1545 nm, and pin array bend loss 3.1 dB.

These properties are within the desired ranges for a compensating fiber.It will be understood that the agreement between the modeled andmeasured dispersion properties depends upon the accuracy of the model.Broadening the first annular region and raising the relative index ofthe second annular region serves to improve bend resistance, increasethe total dispersion slope, and increase cut off wavelength. The betterbend resistance is achieved at the cost of a slight decrease ineffective area.

EXAMPLE 3

Waveguide fiber in accord with the invention was manufactured and had arefractive index profile as shown in FIG. 4. The target profile is shownas solid line 26 in FIG. 4. The profile as manufactured is shown thedashed line 28. The close tracking between the target profile and themanufactured profile shows good process control. Taking the relativeindex percent values and radius values of the dashed line 28, thecentral segment 2 has Δ₀% of 1.05%, outside radius, 12 of 2 μm, annularsegment 4 has Δ₁l% of −0.42%, outer radius 4.6 μm, and a trapezoidalshape having a linear portion of positive slope that begins at Δ₁% andends at a relative index of −0.33%; second annular segment 6 has aflattened region, 10, of width 0.3 μm and relative index near zero, anda raised index region Δ₂% of 0.4%, center radius 16, 5.3 μm, width 1.0μm, and a symmetrical rounded shape.

The properties of the waveguide fiber were effective area 26 μm², totaldispersion at 1550 nm, −40 ps/nm-km, total dispersion slope at 1550 nmof −0.11 ps/nm²-km, and attenuation at 1550 nm of 0.255 dB/km, in goodagreement with the model.

FIG. 3 shows variations on the embodiment of FIGS. 1 and 2. A layer ofSiO₂ glass 22 can be interposed between the central segment 2 and thefirst annular region 4. The index of refraction of this layer can beslightly above that of SiO₂ due to dopant diffusion from adjacentsegments during manufacturing. The width of the layer is no greater than1.5 μm. In addition, a portion of the clad layer 20 adjacent the secondannular region 6, may be designed to have a refractive index less thanthat of SiO₂, as shown by dashed line 24. The thickness of clad portion24 is less than 20 μm. Layers 22 and 24 provide two additionalparameters to adjust in achieving the desired waveguide fiberproperties. Further, the presence of a clad portion 24 serves toincrease bend resistance. The buffer layer 22 serves to decreasediffusion of index increasing material from central segment 2 into firstannular segment 4.

EXAMPLE 4

An additional refractive index profile was modeled using the Δ% valuesand radius values shown in FIG. 12. Central segment 2 can be describedby the equation of an α-profile where α is about 1.47. The relativeindex Δ₀% is 1.08% and the central segment radius is 2.86 μm. Theprofile can be made to exhibit the desired for a range of α values. Forexample a range of α values from 1 to 3 can be used. Annular segment 4has Δ₁% of −0.353%, outer radius 4.9 μm, and an α-profile shape; secondannular segment 6 has a flattened region, 10, of width 2.5 μm andrelative index near zero, and a raised, flattened index region havingΔ₂% of 0.26%, center radius, 8.95 μm, width 2.9 μm.

The modeled properties of this profile are effective area 34.1 μm²,total dispersion slope −0.115 ps/nm²-km at 1550 nm, total dispersion −32ps/nm-km at 1550 nm, attenuation 0.215 dB/km at 1550 nm, fiber cut offwavelength 2070 nm, and pin array bend loss 6.58 dB.

A number of waveguide fibers were made in accord with the invention andtheir attenuation, total dispersion, and total dispersion slope measuredat 1550 nm. Results of the measurements are shown in FIG. 5. The totaldispersion ranged from about −34 ps/nm-km to −47 ps/nm-km. Over thistotal dispersion range, the total dispersion slope remained essentiallyconstant at −0.10 ps/nm²-km, as can be seen from points 30 in FIG. 5.The attenuation at 1550 remained within a range of about 0.24 dB/km to0.33 dB/km over this total dispersion range as shown by points 32 inFIG. 5. The data shows the refractive index profile in accord with theinvention to be relatively easily and reproducibly manufactured.

A telecommunications link was modeled over a wavelength range of 1500 nmto 1600 nm using measured properties of both the positive andcompensating negative total dispersion single mode optical waveguidefibers. Curve 40 of FIG. 6 shows the total dispersion of thecompensating fiber as varying between −36 ps/nm-km and −46 ps/nm-km overthe 1500 nm to 1600 nm range. Curve 42 of FIG. 7 shows the totaldispersion slope of the compensating fiber remains within a range −0.09ps/nm²-km to −0.11 ps/nm²-km over this wavelength range.

Properties of the fiber to be compensated, a standard single modeoptical waveguide fiber as described above, are shown in FIGS. 9 and 10.Curve 44 of FIG. 9 shows the total dispersion of the standard fiber overthe specified wavelength range varies linearly from 16 ps/nm-km at 1500nm to 22 ps/nm-km at 1600 nm. Curve 46 of FIG. 10 shows the totaldispersion slope of the standard fiber over the specified wavelengthrange varies linearly from 0.063 ps/nm²-km at 1500 nm to 0.054 ps/nm²-kmat 1600 nm. A comparison of curve 40 of FIG. 6 to curve 44 of FIG. 9shows the compensation fiber has total dispersion which mirrors thetotal dispersion of the positive dispersion fiber over the wavelengthrange. The total dispersion magnitude of the compensating fiber is abouttwice that of the standard fiber over the wavelength range. A comparisonof curve 42 of FIG. 7 to curve 46 of FIG. 9 shows an analogousrelationship between the respective slopes over the wavelength range ofthe compensating waveguide fiber and the standard waveguide fiber.

A 44 km system was modeled using the standard and dispersioncompensating waveguide fiber in a length ratio of 2 to 1. The modelingresults are shown in FIG. 8. Curve 34 in FIG. 8 shows the dispersionversus wavelength of the standard fiber. Curve 36 in FIG. 8 shows thedispersion versus wavelength of the compensating fiber. The dispersionversus wavelength of the 44 km link is seen in curve 38 of FIG. 8 to beessentially constant about −2 ps/nm-km. Essentially constant meansdispersion slope magnitude of the link is less than or equal to 0.01ps/nm²-km. The fiber made in accord with the invention provides exactcompensation of the standard fiber over the entire wavelength range 1500nm to 1600 nm.

Further modeling of waveguide fibers made in accord with the inventionwas carried out to determine the relative benefits of increasing ordecreasing the compensation ratio, that is, the ratio of the length ofstandard fiber to the length of compensating fiber in the link.Compensating waveguide fibers were designed for use in a modeled link ata 1:1, 2:1, and 3:1 length ratio. Model results are shown in FIG. 11.The horizontal axis is the compensation ratio, the number 1 meaning alength ratio of 1:1. 2 a length ratio of 2:1, and 3 a length ratio of3:1. The vertical axis shows normalized power, which is defined as lightpower P multiplied by the non-linear refractive index coefficient n₂ anddivided by the effective area A_(eff). That is, normalizedpower=Pn₂/A_(eff). For three choices of standard fiber attenuation, the2:1 ratio provided for the lowest normalized power. Curve 48 of FIG. 11,corresponding to a standard fiber attenuation of 0.20 dB/km, has aminimum of about 0.135 normalized units at the 2:1 compensation point.Curve 49 of FIG. 11, corresponding to a standard fiber attenuation of0.185 dB/km, has a minimum of about 0.13 normalized units at the 2:1compensation point. Curve 50 of FIG. 11, corresponding to a standardfiber attenuation of 0.175 dB/km, has a minimum of about 0.115normalized units at the 2:1 compensation point. This indicates that thenon-linear dispersion effects such as modulation instability, four wavemixing, and cross phase modulation will be lowest for compensatingfibers exhibiting a total dispersion and total dispersion slope thatresults in a 2:1 length ratio in the link, as is the case for thecompensating fibers of the invention. The optimum length ratio lies inthe range of 1:1 to 3:1.

It will be apparent to those skilled in the art that variousmodifications and variations of the present invention can be madewithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention include the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

We claim:
 1. A telecommunications link comprising: a transmitter forgenerating light signals; a receiver for receiving the light signals; atleast two lengths of optical waveguide fiber optically joined in seriesto form a link length, the link length having a first end opticallycoupled to said transmitter and a second end optically coupled to saidreceiver to transport the light signals therebetween; wherein, a firstone of said at least two lengths of optical fiber has a positive totaldispersion and a positive total dispersion slope, and a second one ofsaid at least two lengths has a refractive index profile selected toprovide a negative total dispersion and negative total dispersion slope,wherein, the first and second lengths are selected such that said linkhas a total dispersion of magnitude less than 10 ps/nm-km, and a totaldispersion slope of magnitude less than 0.01 ps/nm²-km, over apre-selected wavelength range.
 2. The telecommunications link of claim 1wherein the pre-selected wavelength range is 1280 nm to 1650 nm.
 3. Thetelecommunications link of claim 1 wherein the pre-selected wavelengthrange is 1500 mn to 1650 nm.
 4. The telecommunications link of claim 1wherein the first one of the at least two lengths is longer than thesecond one of at least two lengths.
 5. The telecommunications link ofclaim 4 wherein the first one of the at least two lengths and the secondone of the at least two lengths are present in the link in a ratio inthe range from 1:1 to 3:1.
 6. The telecommunications link of claim 1wherein at 1550 nm the total dispersion of the first one of the at leasttwo lengths is in the range of 8 ps/nm-km to 25 ps/nm-km and the totaldispersion slope is in the range of 0.05 ps/nm²-km to 0.085 ps/nm²-km.7. The telecommunications link of claim 6 wherein at 1550 nm the totaldispersion of the second length of the at least two lengths is in therange of −30 ps/nm-km to −60 ps/nm-km and the total dispersion slope isin the range of −0.09 ps/nm²-km to −0.18 ps/nm²-km.
 8. Thetelecommunications link of claim 7 wherein the link length is not lessthan 40 km, the pre-selected wavelength range is 1500 nm to 1650 nm, thetotal dispersion magnitude over the range 1500 nm to 1650 nm is lessthan 4 ps/nm-km, and the total dispersion slope magnitude over thewavelength range 1500 nm to 1650 nm is less than 0.01 ps/nm²-km.
 9. Thetelecommunications link of claim 8 wherein the total dispersion slopemagnitude of the link over the range 1500 nm to 1650 is less than 0.005ps/nm²-km.
 10. The telecommunications link of claim 8 wherein the totaldispersion of said link is negative.
 11. The telecommunications link ofclaim 1 wherein the first one of said at least two lengths is opticallycoupled to the transmitter.
 12. The telecommunications link of claim 11further including a third length of waveguide fiber having positivetotal dispersion and positive total dispersion slope, wherein said thirdlength is optically coupled to the receiver.
 13. The telecommunicationslink of claim 1 wherein the refractive index profile of the second oneof said at least two lengths has a core region surrounded by and incontact with a clad layer, wherein said core region includes at leastthree segments, a central segment, a first annular segment surroundingthe central segment, and a second annular segment surrounding the firstannular segment.
 14. A telecommunications link, comprising: atransmitter for generating light signals; a receiver for receiving thelight signals; at least two lengths of optical waveguide fiber opticallyjoined in series to form a link length, the link length having a firstend optically coupled to said transmitter and a second end opticallycoupled to said receiver to transport the light signals therebetween;wherein, a first one of said at least two lengths of optical fiber has apositive total dispersion in the range of 8 ps/nm-km to 25 ps/nm-km at1550 nm and a positive total dispersion slope in the range of 0.05ps/nm²-km to 0.085 ps/nm²-km at 1550 nm, and a second one of said atleast two lengths has a refractive index profile having a core regionsurrounded by and in contact with a clad layer, wherein said core regionincludes at least three segments, a central segment, a first annularsegment surrounding the central segment, and a second annular segmentsurrounding the first annular segment and the refractive index profileis selected to provide a negative total dispersion in the range of −30ps/nm-km to −60 ps/nm-km at 1550 nm and the total dispersion slope inthe range of −0.09 ps/nm²-km to −0.18 ps/nm²-km at 1550 nm, wherein thefirst and second lengths are selected such that said link has a totaldispersion of magnitude less than 10 ps/nm-km, and a total dispersionslope of magnitude less than 0.01 ps/nm²-km, over a pre-selectedwavelength range between 1280 nm to 1650 nm.